RCMF Donations

Enjoy using RCMF? How about a wee donation to help us keep you in the style to which you've become accustomed?

Welcome to RCMF. Please login or sign up.

July 07, 2020, 13:43:21 pm

Login with username, password and session length

Futaba 14 SG ( or others)

Started by Skyleader, September 17, 2018, 09:02:19 am

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.


I heard on a recent YouTube build the operator said that he had altered the control down from 100 to 50 percent on the TX end point movement. He also said that it was better to move controls mechanically, as reducing the end point like this caused  a 'loss of resolution' with that control ?

What does the team think ???

'Dont just stand there; get one up!!'


Basically the transmitter has a resolution of so many steps, say 100% = 1000 steps, so if you dial down the movement to 50% then full movement of the stick is 500 steps, so you have half the resolution on the servo movement for the equivalent stick deflection.

Plus if you move the control rod inwards on the servo then the applied force goes up (force = torque/distance from pivot) so the servo will be working less hard and able to hold position with less work. Same applies if you move the control rod out on the control surface horn, just that this time the torque applied to the control surface = force x distance from the pivot.


September 17, 2018, 11:13:23 am #2 Last Edit: September 17, 2018, 11:15:06 am by PDR
In principle he's right. The solution with the maximuum of physical movement at the servo will give the greatest resolution, have the lowest backlash at the control surface and will give the greatest torque*.

The only real benefit of using restricted movement is that it increases the apparent surface speed (if you set the end-point to 50% then a 0.1sec/60deg servo will get to full deflection in 0.05sec. When I was in FAI-F3D pylon racing we used to use this approach on the elevator servos to get the fastest cornering response by running as little as 20% rate at race speed, but I think the actual benefit was imaginary, because the theoretical benefit ignores servo acceleration and deceleration times, and the detail that seros only deliver their claimed response speeds with no load.

There are a couple of other cases where it can be a good idea to restict the servo movement rather than adjust the linkage mechanics:

1. In some ARTFs the linkages are pre-installed and it's not possible to move the link on the control horn or the servo end without getting binding.
2. Related to 1 is the case where it may be preferable (due to linkage geometry and available space) to install linkages that will have minimal lateral sway to avoid swaying pushrods into things, or bending cables/snakes.
3. If you are using linear servos (pretty rare these days, other than the sub-micro ones in E-flite UM/UMX models) where you have no choice on servo throw, and may have limited options on horn positions.
4. These days things like throttles or retract air valves, where a precise movement is desired, can be best achieved by dialing the end-points accordingly. I mean we managed without back in the 70s and 80s, but we were preper modellers in those days and today's ARTF-jockeys can't handle that kind of problem...

[OK so that was 4 rather than a couple - I'm an Engineer, and budget-adherence is a concern for accountants, project managers and similar lesser-minds]

You will notice that I haven't included mechanical retracts in this list. As far as I'm concerned retracts should be driven by 180deg servos so that yo get geometric lock in both up and down positions. Getting the right pushrod movement with a 180deg servo is such a trivially easy task that I would expect even the rawest of ARTF-jockeys to be able to achieve it without breaking a sweat.

Having said all that...

These days the general tendency is to massively over-servo models. Back in the day we used to drive both ailerons on fast 60-size pattern ships with a single Futaba FD16 (a 1.8kg-cm servo). Also most modern servos are so powerful that we choose a specific servo not for its torque by for the strength of its gear chain. A futaba FD16 was "man emnough", but the modernish equivalent would be smaller than a hitek HS81, and I wouldn't trust its gears in the same job. So if you are using this approach it may well be that you literally don't care about the disadvantages, because your servo is mahoosively more powerful than is actually needed and you can afford the losses. But you still need to pay attention to linkage backlash al smaller control movements.Remember that the linkage option with the GREATEST pushrod/cable movement will always have the least backlash.

€0.00006 supplied,


* Some people have trouble grasping this. If you have two identical setups that achieve the same angular movement of the control surface, but one uses smaller servo movement due to end-points or rates, then the one which has more servo-arm movement will need more rotations of the servo motor to get there and thus has a lower gear ratio bewteen the servo motor and the surface. Therefore of the motor torque and the surface movement are the same the available force to drive the control surface MUST be higher. QED
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...